gen2-migration lock fails: AppSync @model nested stack exceeds CloudFormation 51,200-byte templateBody limit

[kunjabijukchhe]

How did you install the Amplify CLI?

npm

If applicable, what version of Node.js are you using?

v25.4.0 (npm 11.7.0)

Amplify CLI Version

14.5.0

What operating system are you using?

macOS 26.5.1 (Apple Silicon)

Did you make any manual changes to the cloud resources managed by Amplify? Please describe the changes made.

No

Amplify Categories

api

Amplify Commands

migrate (gen2-migration)

Describe the bug

Running amplify gen2-migration lock fails during the Planning phase with a
CloudFormation templateBody validation error. One of our @model types
(Document) generates a nested stack whose template body is ~57.8 KB, which
exceeds CloudFormation's hard 51,200-byte limit for an inline TemplateBody.

The migration tooling appears to send this nested stack template inline
(TemplateBody) to a CloudFormation API call (e.g. ValidateTemplate /
CreateChangeSet) during the lock/plan step, rather than uploading it to S3
and referencing it via TemplateURL. Because Gen 1's own amplify push
already uploads these same templates to the deployment bucket and uses
TemplateURL, the project deploys fine on Gen 1 but cannot be locked for
Gen 2 migration.

The Document model is large because it declares 7 @index GSIs
(byOrganization, byUnit, byProperty, byInsurance, byPortfolio,
byReports, byMortgage). Each generated resolver (get/list/CRUD/3
subscriptions + 7 index queries) inlines a sizable VTL request-mapping
template into the nested stack, pushing the single template over 51,200 bytes.

Expected behavior

amplify gen2-migration lock should succeed for any project that currently
deploys successfully with amplify push. Nested stack templates larger than
51,200 bytes should be uploaded to the deployment S3 bucket and referenced via
TemplateURL (the same approach Gen 1 push uses), instead of being passed
inline via TemplateBody.

Reproduction steps

1. Have a Gen 1 GraphQL API with an @model that has enough @index
   directives / resolvers that its generated nested stack template exceeds
   51,200 bytes (in our case a model with 7 GSIs).
2. amplify gen2-migration assess (passes).
3. Run amplify gen2-migration lock.
4. Planning fails with the validation error below.

GraphQL schema (minimal repro shape)

type Document @model {
  id: ID!
  organizationID: ID! @index(name: "byOrganization")
  unitID: ID! @index(name: "byUnit")
  propertyID: ID! @index(name: "byProperty")
  insuranceID: ID! @index(name: "byInsurance")
  portfolioID: ID! @index(name: "byPortfolio")
  reportsID: ID! @index(name: "byReports")
  mortgageID: ID! @index(name: "byMortgage")
}

Log output

→ Planning failed

🛑 1 validation error detected: Value '{"Parameters":{ ... <Document nested
stack template, ~57,843 bytes> ... "transformerrootstackDocumentDocumentTable193B08C2Ref":
{"Value":{"Ref":"DocumentTable"}}}}' at 'templateBody' failed to satisfy
constraint: Member must have length less than or equal to 51200

From ~/.amplify/logs:

amplify gen2-migration core lock
...
error : 1 validation error detected: Value '{...}' at 'templateBody' failed to
satisfy constraint: Member must have length less than or equal to 51200
UnknownFault: 1 validation error detected: Value '{...}' at 'templateBody' ...

The offending nested stack template measures 57,843 bytes (> 51,200).

Additional information

• Gen 1 amplify push succeeds for the same project — it uploads these
  templates to the deployment bucket and uses TemplateURL.
• Workaround attempts: none successful from the user side, since the template
  size is generated by the transformer based on the number of indexes/resolvers.
• IDs (account, identity pool, user pool, table ARNs) redacted from the log
  excerpt above.

[kevinxu3]

+1 any updates on this

[sharonyajain]

Thanks for the detailed report, and for the minimal repro schema — that made this easy to trace.

This is a real bug: in 14.5.0, the gen2-migration lock planning step sent nested-stack templates to CloudFormation inline via TemplateBody, so a large AppSync @model stack (yours was ~57.8 KB with 7 @index GSIs) blows past CloudFormation's hard 51,200-byte limit. As you noted, Gen 1 amplify push avoids this by uploading templates to the deployment bucket and referencing them with TemplateURL.

This is fixed by #14884, which reworked the migration tooling so it uploads every template to the deployment S3 bucket and passes TemplateURL — the same approach Gen 1 push uses — removing the inline size ceiling. The fix shipped in @aws-amplify/cli@14.5.1 (released just after you filed this).

Could you upgrade to the latest CLI (14.5.1 or newer) and retry amplify gen2-migration lock? It should now get past the Planning phase for the Document model. Please let us know how it goes and we'll close this out.

npm install -g @aws-amplify/cli@latest
amplify gen2-migration lock

@kevinxu3 — same ask for you if you're hitting this on 14.5.0.

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